A dynamoelectric machine of the permanent magnet type, such as a permanent magnet generator, generally includes a rotor assembly rotatably mounted within a stationary outer stator. The rotor assembly includes a plurality of annularly spaced permanent magnets. Usually, the magnets are equally spaced and of an even number to provide alternating polarity magnetic means around the periphery of the rotor assembly. The stator usually includes windings arranged to magnetically couple with the rotor poles. During rotation of the rotor assembly, the flux of the permanent magnets induce an electromotive force in the stator coils.
When the permanent magnets are disposed around the radial periphery of the stator assembly, it is common to locate the permanent magnets in radially outwardly opening slots annularly spaced about a rotor hub. A containment shell surrounds the hub and the spaced permanent magnets. There is a continuing problem in designing such rotor assemblies, particularly in the design of the containment shell and involving the force parameters interacting between the components, including considerations of varying forces on the magnets and the hub portions between a static condition and high speed rotational conditions during operation of the dynamoelectric machine.
For instance, the hub of the rotor assembly may have outwardly diverging partition portions separating the permanent magnets, with the magnets being disposed in outwardly converging slots. It has been discovered that the typical design approach results in the root area between the hub slots being subjected to compressive forces, even during operation of the machine at high rotational speeds. This results in inefficient design parameters, particularly in the outer containment shell which often is overdesigned in its strength parameters or simply is made too thick. In aerospace applications, this results in excessive size and weight parameters which are critical in aerospace applications. With such outwardly diverging hub portions between the outwardly converging spaced slots, it would be ideal to somehow place the root areas of the hub portions under tension. Regardless of the slot configuration, it would be desirable to preload the permanent magnets in such a manner that the load is directed only in a radially outward direction toward the containment shell.
This invention is directed to solving the above problems and satisfying the stated needs by providing a preloading system which enables the containment shell to be designed with lower strength requirements and/or thinner dimensions than has been heretofore affordable.